Producing the world's enormous reserves of immobile heavy oil or mobile oil with high viscosity located beyond the reach of open pit mining techniques requires the use of thermal processes. These processes deliver heat to the reservoir in one way or another to heat the oil and mobilize it for production. The dominant thermal processes are:                Steam Assisted Gravity Drainage;        Steam Drive; and        Cyclic Steam Stimulation.        
Steam Assisted Gravity Drainage (SAGD) is a thermal recovery technique widely used for the recovery of extra heavy oils, in which pairs of horizontal injector and producer wells are drilled within the drains roughly 5 m apart. Steam is injected to heat and mobilize the oil, so that it will flow into the producer by gravity drainage assisted by a growing steam chamber. It is the most common thermal recovery technique for extra heavy oil found in oil sands reservoirs.
Steam Drive is another process where steam is continuously injected into dedicated wells (vertical, deviated or horizontal, placed around, or next to, the producers in a predetermined pattern).
Cyclic Steam Stimulation is a single well process, where steam is injected for a certain period of time, then allowed to soak into and heat the oil. Finally, the heated oil and condensed water/steam are produced back through the same well for a period of time.
For all these processes, water is used to generate steam, and the typical water consumption associated with steam generation may be in the order of 3 volumes of water for one volume of produced bitumen, e.g. producing 100,000 b/day of bitumen using a thermal process may require 300,000 b/day (50,000 m3/day) of water. Thermal production may involve consumption of fresh water, but great efforts are made to maximize water recycling: in steam projects, 85% to 95% of the produced water returning from producer wells is recycled via treatment processes.
Steam generation equipment can take various forms that generally include either once through steam generators (OTSG) or conventional boilers. Water of suitable quality is required for feeding the boilers and steam generators.
One of the problems involved in the generation of steam is indeed the presence of mineral contaminants in the water, mainly carbonates, sulfates and silica. As water is heated and converted into steam, the mineral contaminants tend to be left in the steam generator or boiler. Indeed, the steam generator or boiler functions as a distillation unit, taking pure water out as steam and leaving behind concentrated minerals. Scale forms as a result of the precipitation of normally soluble solids that become insoluble in the steam generator or boiler. Scale acts as an insulator, reducing boiler efficiency. Scaling can lead to boiler tube failure due to overheating. For carbonate and bicarbonate species, such phenomena may be detrimental to the application since they may decompose at high temperature producing acidic steam in equilibrium with alkaline water.
The water treatment usually involves heavy processes for de-oiling, silica removal and hardness treatment (hard water is a type of water that has high mineral content with calcium, magnesium metal cations and sometimes other dissolved compounds such as bicarbonates and sulfates). The complexity of the water treatment scheme required depends on the development scheme (whether or not there is a recycling of the produced water to feed the boiler), the feed-water specifications, and the type of boiler.
De-oiling conventionally involves a skim tank, gas flotation and filtration. The de-oiling stage is an essential stage when a recycling scheme is implemented (i.e. when the produced water is used as feed-water in the boiler).
After the de-oiling stage, water should be treated against hardness and silica deposition. This complementary water treatment depends on the type of boiler. Indeed, the boilers are usually classified into two main categories:                80% Quality Steam boilers (OTSG boilers) which produce 20% of water blowdown to be treated; this type of boiler can handle low quality feed water charged in silica, dissolved solids and salts. The associated water treatment is based on lime softening and acid cation exchange.        100% Quality Steam Boilers (conventional drum or Pulverized Coal boiler or Circulating Fluidized Bed boilers) which produce only traces of blowdown. This type of boiler requires more stringent water specifications. The associated water treatment may be based on evaporator technology. It helps to achieve a much higher quality feed water for the boiler than with the lime softening +ion exchange process. The principle of the evaporator is to boil the produced water to generate a stream of vapor (less charged in silica and minerals) and a stream of produced water charged in silica and minerals (which are not evaporated). This non-evaporated produced water is recirculated until it starts to produce waste brine (which is removed).        
Mineral contaminant removal must at any rate be adapted to the type of water used and to the conditions of operation. In particular, when water is recycled, the treatment of silica is often the most prominent concern because silica tends to accumulate in the water cycle especially if the hydrocarbon formation has a high silica content (silica is dissolved in the reservoir by steam injection).
Conversely, there are other instances where silica is not the primary concern (for example because the produced water is not recycled to boiler feedwater and/or because the primary source of feedwater is poor in silica) but where carbonate and/or sulfate ions are a major issue (for example because the primary source of feedwater is rich in carbonate and/or sulfate ions).
In such instances, carbonate and/or sulfate ion removal is conventionally primarily performed by adding one or more chemical substances to the feedwater in order to precipitate the carbonate and/or sulfate ions, and by decanting the precipitated material. However, large quantities of chemical substances are generally required; besides, the decantation time is high and the corresponding settling tanks are very large.
Another option, specifically for carbonate ion removal, consists in acidifying the water so as to convert the carbonate ions to carbon dioxide, and agitating the water so as to degas it, thus eliminating the carbon dioxide. However this option is not feasible in view of the large water tank volumes which would be required for its implementation.
Document WO 2009/029651 teaches to use a ceramic membrane for removing silica from feedwater. Document WO 2009/029653 teaches to eliminate silica by adsorption on mineral species at a high pH, so as to also remove it from feedwater. However, in both cases the process is ineffective for removing large quantities of carbonate or sulfate ions.
There is thus a need for an improved process for removing carbonate and/or sulfate ions from boiler feedwater. In particular, there is a need for a quicker process, using smaller settling tanks and/or less chemical substances.